A domestic digital video tape, which was put to practical use in 1995, is provided with a Co metal magnetic thin layer by a vacuum deposition on one surface of a base film having a thickness of 6 to 7 μm, on the surface thereof a diamond-like carbon layer is coated, and it has one hour recording time as a basic specification (SD specification) in a case of a camera integral type video using a DV minicassette.
This digital video cassette (DVC) is a first digital video cassette in the world for domestic use, and it has merits such as (a) it can record enormous information in spite of small body, (b) image and sound qualities do not deteriorate for years because its signal does not deteriorate, (c) high image and sound qualities can be enjoyed because it does not receive disturbance of noise, and (d) its image does not deteriorate even if dubbing is repeated, and therefore, it is highly evaluated in the market.
Further, DVCAM format of the same tape width (¼ inch), which realized high image quality and high reliability required for business use by using the above-described domestic DV format as the base, using a deposition-type digital video tape, increasing the tape running speed by 1.5 times, and widening the width of the recording track from 10 μm to 15 μm, was developed in 1996. The DVCAM format has been evaluated very highly as a business-use VTR realizing high image quality, high sound quality, small size and light weight and excellent in properties for business use such as dubbing property and high level editorial performance in the fields of companies, productions, cable televisions and video journalists.
As base films of such digital video tapes, films such as the following films have been used.
{circle around (1)} a polyester film which comprises a polyester film, and a discontinuous skin layer contacted on at least one surface of the film containing a polymer blend material and fine particles with a particle diameter of 50 to 500 Å as its main constituent, and in which a water soluble polyester copolymer is contained in the discontinuous skin layer and fine projections are formed on the discontinuous skin layer (for example, JP-B-SHO 63-57238), and{circle around (2)} a composite film in which layer A composed of a thermoplastic resin and layer B composed of a thermoplastic resin containing fine particles are laminated (for example, JP-B-HEI 1-26338)
In such base films, however, problems are liable to occur such as a problem that foreign matters exist in a polyester used for forming a film, a problem that scratches are generated during a process for film formation, and a problem that surface defects are generated by surface contaminated substances adhered during a process for film formation, and the dropout (DO) of magnetic tapes made from the base films is liable to increase.
For the purpose of giving a polyester film for magnetic recording medium which becomes a DVC tape with less dropout, a polyester film for magnetic recording medium is proposed, which is a polyester film having one major surface A provided with fine surface projections with a height of 10 to 50 nm at a number of 3 million to 90 million/mm2, and in which the number of surface projections with a height of 50 to 120 nm existing on the surface A of the polyester film is 40,000/mm2 or less, and the number of surface defects with a height of 120 nm or more is 400/100 cm2 or less (JP-A-2000-25105).
Further, a polyester film for magnetic recording medium is proposed, which is a polyester film having one major surface A provided with fine surface projections with a height of 10 to 50 nm at a number of 3 million to 90 million/mm2, and in which the number of surface projections with a height of 50 to 120 nm existing on the surface A of the polyester film is 1,000/mm2 or less, the number of surface defects is 1,000/mm2 or less, and the sum of the numbers of surface projections and surface defects with a height more than 120 nm is 400/100 cm2 or less (JP-A-2002-50028).
In a magnetic tape made from such a conventional base film, however, although dropout (DO) was decreased as a DVC tape and the number of dropouts for one minute was achieved to be almost zero, in a case where it was used for use of DVCAM capable of recording for three hours as a standard tape, about 10 dropouts were liable to occur in the three hour recording. In a case used for business use, it is desirable that there is no dropout over the entire length of the tape, and it has been strongly required that the number of dropouts is suppressed to be 2 or less for 3 hours, preferably, to be zero. It has been recognized that the DO is caused by surface defects originating from foreign matters in a film with a height of 120 nm or more, which exist on the surface of the base film of the DVCAM magnetic tape, and by surface adhered substances originating from catalyst residuals of polyester precipitated in a process for producing a polyester film, in particular, in an extrusion process.
On the other hand, the domestic digital video tape, which was put to practical use in 1995, is produced by providing a Co metal magnetic thin layer on one surface of a base film by a vacuum deposition, coating a diamond-like carbon layer on the surface thereof, providing a lubricant layer thereon by coating, and providing a back coat layer on an opposite surface of the base film in order to ensure the running property and durability of the tape in a video tape recorder. This digital video tape has a good durability in spite of having a surface further flattened as compared with that of ME tape for Hi8 (deposited tape).
As the base film of such a digital video tape, films such as the following films have been used.
{circle around (3)} a composite film which comprises a polyester film and a discontinuous skin layer brought into contact with at least one surface of the film, and in which fine particles exist in the discontinuous skin layer and on the surface of the skin layer (for example, JP-B-SHO 62-30105),{circle around (4)} a composite film in which layer A composed of a thermoplastic resin and layer B composed of a thermoplastic resin containing fine particles are laminated (for example, JP-B-HEI 1-26338), and{circle around (5)} a film formed with a cover layer containing slipping agent as its main constituent on a surface of a non-magnetic surface side of a flat polyester film (for example, JP-A-SHO 57-195321, JP-B-HEI 1-49116 and JP-B-HEI 4-33273)In such base films, the surface roughness of the metal magnetic layer forming surface side is designed to be further smaller as compared with that in a base film for Hi8ME tape.
However, in the domestic digital video tape thus having a very flat magnetic surface, the electromagnetic conversion property varies very greatly depending on a variation of the surface property of the magnetic surface, and the dropout (DO) property of the obtained tape varies very greatly depending on the influence of foreign matters adhered to a cooling can in a deposition process.
Namely, when the aforementioned {circle around (3)} film is used as a base film for providing a ferromagnetic metal thin layer for a digital video tape by vacuum deposition, although a good high-density magnetic recording property can be obtained, the handling property is not good, and therefore, it is not suitable for mass production. A tape made from the aforementioned {circle around (4)} film has a great dispersion of surface undulation of the magnetic surface of the tape, and there is a problem that the dispersion of the electromagnetic conversion property becomes great. Further, in a case where the aforementioned {circle around (5)} film is used, the cover layer containing lubricant as its main constituent on a surface of a non-magnetic surface side is liable to be chipped or delaminated in a deposition process, in particular, by a cooling can of vacuum deposition, thereby increasing DO.
For the purpose of giving a polyester film for magnetic recording medium, wherein soil adhered to a cooling can in a vacuum deposition process is little, and surface undulation of a magnetic tape obtained by vacuum deposition is small, which can be formed into a magnetic tape having a good electromagnetic conversion property, and which is good in handling property and suitable for mass production, JP-A-HEI 10-172127 proposes a polyester film for magnetic recording medium wherein SRa value of one surface A of a polyester film is in a range of 2 to 4 nm, SRz value thereof is in a range of 10 to 40 nm, SRa value of the other surface B is in a range of 5 to 15 nm, SRz value thereof is in a range of 50 to 250 nm, a cover layer with a good slipping property formed by coating does not exist at a position outside of the surface B, the number of projections with a height of 540 nm or more is in a range of 2 to 20/100 cm2, and a ferromagnetic metal thin layer is provided at a position outside of the surface A.
However, very favorable comments are given to the domestic digital video tape, it is desired that more tapes are provided to the market in the world, and therefore, so as to produce more digital video tapes by a single deposition operation, it has been carried out to increase the length of a base film wound on a roll up to 15,000 m or more, further, to 20,000 m or more, as compared with the conventional length of 12,000 m or less.
Further, increase of the deposition speed at the time of producing a magnetic tape has been carried out, and the production amount of digital video tapes per one day has been increased. In order to increase the deposition speed, a greater amount of Co metal thin layer must be provided to a surface of a base film within a constant time, a quantity of heat escaped from a base film to a cooling can at the time of vacuum deposition must be increased, and the temperature of the base film is liable to go up. Therefore, in a case where a base film disclosed in JP-A-HEI 10-172127 is used and a domestic digital video tape is produced at an increased deposition speed, it has been recognized that decomposed substances of a polyester film are likely to be precipitated from the surface B of the film, the decomposed substances transfer to the side of a deposition surface, the electromagnetic conversion property of a produced digital video tape is not good and dropouts is liable to increase.
As base films for preventing this precipitation of decomposed substances of polyester from the surface B, films such as the following films are proposed.
{circle around (6)} a polyester film in which a cover layer with a thickness of 1 to 10 nm formed by coating is provided at a position outside of the surface B (for example, JP-A-2002-140812),
{circle around (7)} a polyester film which comprises a polyester layer A and a layer B laminated on one surface of the layer A, and in which the content of a cyclic trimer of the polyester of the layer B is 0.8 wt % or less, and the carboxy concentration thereof is 35 equivalent/t or less (for example, JP-A-2002-248723), and{circle around (8)} a polyester film which comprises a polyester layer A and a layer B laminated on one surface of the layer A, and in which the intrinsic viscosity (IV) of the polyester of the layer B is 0.55 or more, and the carboxy concentration thereof is 36 equivalent/t or more (for example, JP-A-2002-248726)
However, in a case where these base films are films with a winding length of 15,000 m or more, when the films immediately after production are used as base films for digital video tapes, the precipitation of decomposed substances of a polyester film from the film surface B at the time of vacuum deposition is little, the electromagnetic conversion property of a produced digital video tape is good and dropouts are few. However, accompanying with expiration of time after film production, foreign matters such as a low-molecular material (an oligomer) of polyester remarkably precipitate onto the surface B over an area from an intermediate position of the winding of the product of the base film to the core portion of the winding. Because the foreign matters such as an oligomer are transferred onto the surface A, after a ferromagnetic thin layer is formed on the layer A by vacuum deposition, the electromagnetic conversion property of the produced digital video tape becomes bad. It has been recognized that this problem is revealed as a phenomenon that, in a case where the lead time from the film production to the vacuum deposition is long to be three months or more, dropouts are liable to increase in a digital video tape produced using such a base film.
Accordingly, it could be helpful to provide a polyester film for magnetic recording medium which can produce a deposition-type magnetic tape for DVCAM having very few dropouts.
Further, it could be helpful to provide a polyester film for recording medium wherein a polyester film of a long roll product with a winding length of 15,000 m or more is used as a base film, deposition is carried out at an increased deposition speed, even if a thermal load applied to the base film at the time of deposition increases, precipitation of decomposed substances of polyester from film surface B does not occur, a cooling can does not soil, and even if the lead time from film production to vacuum deposition becomes long to be three months or more, foreign matters such as oligomer do not precipitate over the entire length of the product from a winding start position, through an intermediate position, to a winding core position of the film roll product, the electromagnetic conversion property of the produced digital video tape is good, and a digital video tape with few dropouts can be produced.